The wireless transmission of power has attracted considerable interest over the past century, ever since Nikola Tesla proposed theories of wireless power transmission in the late 1800s, and can be classified into two broad categories: Wireless Energy Transfer (WET) and Wireless Energy Harvesting (WEH). The former is used for high RF power densities (normally to transfer power from dedicated RF sources over short distances) while the latter relates to the harvesting of the much lower RF power densities that are typically encountered in the urban environment (e.g. from WiFi and mobile phone networks). WEH systems are generally designed to profit from such freely available RF transmissions by employing highly efficient RF-to-DC conversion to supply low-power devices.
The efficiency, η, of an RF-to-DC converter is defined as:
                              η          =                                    P              OUT                                      P              IN                                      ,                            (        1        )            where PIN is the input RF power and POUT is the output DC power.
Owing to the very low level of ambient RF transmissions (where PIN is typically zero dBm or less), it is highly desirable for RF-to-DC converters to operate as efficiently as possible, preferably for different RF power sources and/or DC loads. Although some practical RF-to-DC converters have been developed, these employ complex filter designs having several lumped components, which limit their efficiency.